1. Field of the Invention
The invention relates to a magnetically tunable filter.
2. Related Technology
Magnetically tunable filters are employed, for example, as variable bandpass filters in spectrum analyzers and network analyzers, the desired resonant frequency being adjusted by means of an external variable magnetic field.
U.S. Pat. No. 4,888,569 discloses a variable bandpass filter for frequencies within a frequency range of at most one waveguide band, for example 50-75 GHz, with four resonator spheres. The variable bandpass filter comprises an input waveguide, an output waveguide and a transfer waveguide, which are configured for the propagation of a TE10 wave mode. The end of the input waveguide terminated by a short-circuit wall, the start of the output waveguide which is likewise provided with a short-circuit wall, and the transfer waveguide fitted below the input waveguide and the output waveguide in the direction of the externally applied homogeneous magnetic field, is arranged during operation between two magnet poles which supply a variable magnetic field for adjusting a resonant frequency. In the direction of the wave propagation, the input waveguide and output waveguide have a rectangular profile which has a much smaller cross-sectional area in the coupling region than at the connection flange. The coupling region of the variable bandpass filter comprises the four resonator spheres, fitted close to a short-circuit wall, and respectively the tapered ends of the input waveguide and of the output waveguide, as well as the transfer waveguide with a constant cross-sectional area.
A disadvantage of the variable bandpass filter described in U.S. Pat. No. 4,888,569 is that in the resonant case the field distribution of the wave to be extracted is unfavourable in the coupling region, since it is guided in a waveguide whose profile is reduced towards the coupling region perpendicularly to the propagation direction of the wave to be extracted. This causes undesired reflections which are destructively superposed and therefore reduce the amount of energy transported by the incoming wave. This effect also pertains to the outgoing wave in the output waveguide, which now has a defined frequency, so that overall the insertion loss in relation to the entry of the input waveguide and the exit of the output waveguide is increased since the field distributions in the coupling region are perturbed owing to the tapering geometry of the waveguides.
Another disadvantage is the limited bandwidth of the waveguide concept.